Switching power source device and control IC which are capable of performing constant power control

ABSTRACT

The present invention is a switching power source device which converts AC power of an AC power source into DC power and outputs the DC power, the device including: a rectifying-smoothing circuit configured to output a rectified-smoothed voltage signal obtained by rectifying and smoothing an AC voltage of the AC power source; a transformer having a primary winding, a secondary winding, and an auxiliary winding; a switching element connected to the primary winding of the transformer; and a control circuit configured to turn the switching element on and off based on a voltage signal which is based on an average value of current flowing through the switching element and the rectified-smoothed voltage signal from the rectifying-smoothing circuit.

BACKGROUND OF INVENTION

1. Technical Field

The present invention relates to a switching power source device and acontrol IC for lighting LEDs which are capable of performing constantpower control.

2. Background Art

FIG. 1 is a configuration block diagram showing a conventional LEDlighting device described in Japanese Patent Application Publication No.2006-210836. In FIG. 1, an EMI filter 1 is connected between bothterminals of an AC power source AC, and an output terminal of the EMIfilter 1 is connected to an input terminal of a full-wave rectifyingcircuit DB configured to rectify an AC voltage of the AC power sourceAC. An output terminal of the full-wave rectifying circuit DB isconnected to a capacitor Cin. The full-wave rectifying circuit DB andthe capacitor Cin form an input-side rectifying-smoothing circuit. Astage subsequent to the capacitor Cin is formed by a flyback converterhaving a transformer TR, a switching element Q1, a control integratedcircuit (IC) 10, and an LED-group load device 3.

The transformer TR has a primary winding W1, a secondary winding W2, andan auxiliary winding W3. The secondary winding W2 and the auxiliarywinding W3 are wound to have reversed phase compared to that of theprimary winding W1. The switching element Q1 is formed with a MOSFET,and is connected to the primary winding W1 of the transformer TR anddriven by the control IC 10 configured to generate an on/off drivesignal.

An output-side rectifying-smoothing circuit formed by a series circuitof a diode D1 and a capacitor C1 is connected between both ends of thesecondary winding W2 of the transformer TR, and the LED-group loaddevice 3 having series-connected LED1 to LEDn is connected between bothterminals of the capacitor C1. A rectifying-smoothing circuit formedwith a series circuit of a diode D2 and a capacitor C2 is connectedbetween both ends of the auxiliary winding W3 of the transformer TR, anda voltage generated at the capacitor C2 is supplied as a power sourcevoltage Vcc for the control IC 10.

To perform constant current control on a load current flowing throughthe LED-group load device 3, a constant current circuit (CC circuit) isformed, and a detected load current is inputted, as a feedback signalFB, to a current-control error amplifier (not shown) in the control IC10 so that the load current may have a predetermined average currentvalue.

However, when the constant current control is performed on currents forLEDs whose forward voltages VF largely vary, power inputted to the LEDsvaries, causing variations in the amount of luminous flux amongindividual lighting devices.

In some cases, to suppress the variations in the amount of luminousflux, constant power control is performed on the LED loads. In theconstant power control on the LED loads, a constant current IF isdecreased when the forward voltage VF of an LED is high while theconstant current IF is increased when the forward voltage VF is low. Inthis case, a computation of VF×IF=constant needs to be performed bycircuitry. For example, VF×IF=constant can be achieved by using amultiplier or the like to generate an FB signal having information forconstant power.

When VF×IF=constant is achieved using the multiplier or the like, asingle path is formed for the FB signal by placing the multiplier or thelike at the secondary side of the transformer TR, but a photo coupler orthe like is needed to form an insulating power source. Thus, the circuitconfiguration becomes complicated. Moreover, it is difficult with aconventional circuit technology to perform the constant power controlwith a simple circuit other than the multiplier or the like.

An objective of the present invention is to provide a switching powersource device and a control IC which are inexpensive and small and arecapable of performing constant power control without a multiplier and aphoto coupler and reducing variations in the amount of luminous fluxeven when the forward voltages VF vary.

SUMMARY OF INVENTION

To overcome the above problem, the present invention provides aswitching power source device which converts AC power of an AC powersource into DC power and outputs the DC power, the device including: arectifying-smoothing circuit configured to output a rectified-smoothedvoltage signal obtained by rectifying and smoothing an AC voltage of theAC power source; a transformer having a primary winding, a secondarywinding, and an auxiliary winding; a switching element connected to theprimary winding of the transformer; and a control circuit configured toturn the switching element on and off based on a voltage signal which isbased on an average value of current flowing through the switchingelement and the rectified-smoothed voltage signal from therectifying-smoothing circuit.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a configuration block diagram showing a conventional LEDlighting device.

FIG. 2 is a circuit diagram showing the configuration of a switchingpower source device of Embodiment 1 of the present invention.

FIG. 3 is a diagram showing ILED current values for achieving constantpower, for each number of LED loads in the switching power source deviceof Embodiment 1.

FIG. 4 is a diagram showing errors in current, voltage, and power foreach number of LED loads and each input voltage in the switching powersource device of Embodiment 1.

FIG. 5 is a circuit diagram showing a configuration of an LED lightingdevice performing constant current control.

FIG. 6 is a diagram showing an average drain current value for eachinput voltage, observed when constant power output characteristics areobtained by a current mode PWM method.

FIG. 7 is a circuit diagram showing a method of extracting and combiningan OCP signal in the switching power source device of Embodiment 1.

FIG. 8 is a circuit diagram showing the inside of a control IC in aswitching power source device of Embodiment 2.

FIG. 9 is a circuit diagram showing the inside of a control IC in aswitching power source device of Embodiment 3.

FIG. 10 is a circuit diagram showing the inside of a control IC in aswitching power source device of Embodiment 4.

DESCRIPTION OF EMBODIMENTS

Next, switching power source devices according to embodiments of thepresent invention are described with reference to the drawings. Eachswitching power source device of the present invention achieves constantpower control by using a switching current at a primary side of atransformer as a FB signal to thereby eliminate a constant-powerfeedback circuit at a secondary side of the transformer. Thus, there canbe provided an inexpensive, small switching power source device havingsmall variations in the amount of luminous flux even when forwardvoltages VF vary, without needing a constant power feedback circuit, aphoto coupler, and the like.

Embodiment 1

FIG. 2 is a circuit diagram showing the configuration of a switchingpower source device of Embodiment 1 of the present invention. Theswitching power source device of Embodiment 1 shown in FIG. 2 is appliedto an LED lighting device. Note that the switching power source deviceof Embodiment 1 may also be applied to a device other than the LEDlighting device.

The LED lighting device shown in FIG. 2 is different from that shown inFIG. 1 in that it performs constant power control without the constantpower feedback circuit on the secondary side of the transformer TR shownin FIG. 1 but with resistors R1 to R4, diodes D3 and D4, and a capacitorC3 provided additionally.

The resistor R1 is connected at one end to an output terminal of afull-wave rectifying circuit DB, to one terminal of the capacitor Cin,and to one end of a primary winding W1 of a transformer TR, andconnected at the other end to one end of the resistor R2 and to one endof the resistor R3. The other end of the resistor R3 is connected to afeedback terminal FB of a control IC 10 and to a cathode of the diodeD3.

The other end of the resistor R2 is connected to one terminal of acapacitor C2 and to one end of a resistor Rocp for overcurrentprotection (OCP) connected to a source of a switching element Q1. Theother end of the resistor Rocp is connected to one end of the capacitorC3 and a cathode of a diode D4. The other end of the capacitor C3 and ananode of the diode D4 are connected to an anode of the diode D3 and oneend of the resistor R4.

A cathode of the diode D3 is connected to the other end of the resistorR3 and to the feedback terminal FB of the control IC 10, and the otherend of the resistor R4 is connected to a cathode of the diode D2, theother end of the capacitor C2, and a power source terminal Vcc of thecontrol IC 10.

A control circuit 5 has the switching element Q1, the control IC 10, aD/ST terminal, an S/OCP terminal, a GND terminal, a Comp terminal, theFB terminal, and the Vcc terminal.

Other configurations shown in FIG. 2 are the same as those shown inFIG. 1. The same portions are given the same reference numerals, and arenot described again here.

Next, before describing operation of the switching power source deviceof Embodiment 1 shown in FIG. 2, a description is given of an LEDlighting device shown in FIG. 5 performing constant current control. TheLED lighting device shown in FIG. 5 performing constant current controladjusts a current value so that power inputted to LEDs may be constanteven if the LEDs are increased or decreased in number (constant powercontrol). FIG. 6 shows average values of drain current of the switchingelement Q1 in a case where constant power output characteristics areobtained in a current mode PWM. The average values of drain currentshown in FIG. 6 are obtained with an AC input voltage Vin (AC) beingchanged.

For cases of nine, ten, and eleven LEDs connected in series, when powerinputted to the LEDs are made constant (constant power control) byincreasing/decreasing a current value, the average values of draincurrent of the switching element Q1 show almost constant values, asshown in FIG. 6, irrespective of the number of series-connected LEDs.

A curve inversely proportional to the AC input voltage (curve formed bythe average values of drain current shown in FIG. 6) is a signal voltagesubjected to the constant power control. If the signal voltage is usedas a feedback terminal FB signal as it is, the power control performedis proportional to the AC input voltage. Thus, the signal voltage needsto be subjected to some correction. Specifically, a constant FB voltagevalue can be obtained by superimposing, as an input correction voltage,a signal proportional to a voltage obtained by rectifying and smoothingthe AC input voltage, on the average values of drain current shown inFIG. 6. Thus, presumably, the constant power control can be performedusing the average values of drain current of the switching element Q1.

A drain current of the switching element Q1 is converted into a voltageby the current detection resistor Rocp and averaged. There is an optimumvalue for the proportion of that voltage to the FB voltage, andadjustment is needed so that the above-described value obtained by thesuperimposition of, as the input correction voltage, the signalproportional to the voltage obtained by rectifying and smoothing the ACinput voltage may be an optimum value.

For the reasons above, the constant power control can be performed byinputting a value obtained by combining the average values of draincurrent of the switching element Q1 with the AC input correction voltageinto the feedback terminal FB of the control IC 10.

FIG. 7 shows a method for extracting and combining an OCP signal in theswitching power source device of Embodiment 1.

In FIG. 7, the drain current of the switching element Q1 from the S-OCPterminal of the control circuit 5 is converted into voltage informationby the resistor Rocp. Then, the high-speed diodes D3 and D4 performimpedance conversion on the FB terminal. In this case, voltage generatedbetween both ends of the resistor Rocp is transmitted to the FB terminalvia the diodes D3 and D4. Further, since the diodes D3 and D4 are biasedby a Vcc voltage via the resistor R4, the voltage is increased by thediode D4 at the anode thereof by the amount of the forward voltage VF.

A terminal voltage of the anode of the diode D4 is decreased by thediode D3 at the cathode thereof by the amount of the forward voltage VF,and is transmitted after being returned, at the FB terminal, to avoltage level of the resistor Rocp. Further, an AC input correctionvoltage is applied via the resistor R3 to the cathode of the diode D3,the AC input correction being a DC voltage obtained by rectifying an ACvoltage. Specifically, a voltage obtained by combining a voltagecorresponding to an average value of drain current of the switchingelement Q1 with the AC input correction voltage is inputted to thefeedback terminal FB of the control IC 10. Thus, the constant powercontrol can be performed.

As described, according to the switching power source device ofEmbodiment 1, a voltage signal based on the average value of a currentflowing through the switching element Q1 and a voltage signal rectifiedand smoothed by the rectifying-smoothing circuit DB, Cin are inputted tothe feedback input terminal FB of the control IC 10. Thus, the feedbackvoltage value is constant irrespective of the input voltage value,thereby enabling the constant power control. Hence, constant powercontrol can be performed without needing a multiplier and a photocoupler, allowing provision of an inexpensive, small switching powersource device causing less change in the amount of luminous flux evenwhen there are variations in the forward voltage VF.

FIG. 3 shows line regulation for each number of LED loads in theswitching power source device of Embodiment 1. FIG. 3 shows lineregulation for each number of LED loads in a case where a voltage signalbased on the average values of current flowing through the switchingelement Q1 and a voltage signal rectified and smoothed by therectifying-smoothing circuit DB, Cin are inputted to the feedback inputterminal FB of the control IC 10 of the switching power source device ofEmbodiment 1.

As can be seen in FIG. 3, the smaller the number of LED loads, thesmaller a voltage VLED between both ends of each LED but the larger acurrent ILED flowing through the LED. Then, the larger the number of LEDloads, the larger the voltage VLED between both ends of the LED, but thesmaller the current ILED flowing through the LED.

FIG. 4 shows how much current, voltage, and power change when the numberof LED loads in the switching power source device of Embodiment 1 ischanged. As can be seen in FIG. 4, when the number of LED loadsincreases or decreases, the voltage VLED and the current ILED change inreverse proportion to each other. Thus, the power is constant, and for10% fluctuations of an LED load factor, change in the power is within±2%.

Embodiment 2

FIG. 8 is a circuit diagram showing the inside of a control IC of aswitching power source device of Embodiment 2. A control IC 10 a shownin FIG. 8 has a switching element Q1, an AND circuit 11, a PWMcomparator 12, a triangular-wave circuit 13, a current-control erroramplifier 14, a capacitor C6, and an overcurrent protection circuit OCP.

As an FB signal, a value obtained by combining an S/OCP signal shown inFIG. 7 (an average value of drain current of the switching element Q1)and a DC voltage obtained by AC rectification is inputted to an FBterminal shown in FIG. 8. The current-control error amplifier 14 isconfigured to amplify an error voltage between the FB signal from the FBterminal and a reference voltage Vref2, and output the amplified errorvoltage to a non-inverting input terminal of the PWM comparator 12. ThePWM comparator 12 is configured to compare a triangular-wave signal fromthe triangular-wave circuit 13 with the amplified error voltage from thecurrent-control error amplifier 14 and thereby generate a pulse signalfor turning the switching element Q1 on and off. This pulse signalchanges the ON duration according to the amplified error voltage.

The overcurrent protection circuit OCP is configured to output level Lto the AND circuit 11 when a voltage generated by a current flowingthrough the switching element Q1 exceeds a reference voltage Vref1. TheAND circuit 11 is configured to turn the switching element Q1 off inresponse to level L outputted from the overcurrent protection circuitOCP. The AND circuit 11 turns the switching element Q1 on and off inresponse to the on/off pulse signal from the PWM comparator 12.

According to the control IC 10 a configured as such, a switching elementQ1 is provided inside the control IC 10 a, and the FB signal obtained bycombining an S/OCP signal (an average values of drain current of theswitching element Q1) and a DC voltage obtained by AC rectification isinputted to the FB terminal. The control IC 10 a can thus beimplemented.

Embodiment 3

FIG. 9 is a circuit diagram showing the inside of a control IC of aswitching power source device of Embodiment 3. The control IC shown inFIG. 9 does not have the FB terminal of the control IC shown in FIG. 8and configured to input an average value of drain current of theswitching element Q1 to an inverting input terminal of a current-controlerror amplifier 14 via a filter 15 and input a DC voltage obtained by ACrectification to a Comp terminal.

The filter 15 is configured to remove noise contained in a voltagecorresponding to an average value of drain current of the switchingelement Q1 and outputs the voltage to the inverting input terminal ofthe current-control error amplification 14. The current-control erroramplification 14 is configured to amplify an error voltage between theoutput of the filter 15 and a reference voltage Vref2.

A PWM comparator 12 is configured to generate a pulse signal bycomparing a triangular-wave signal from a triangular-wave circuit 13with an added output obtained by adding the output of thecurrent-control error amplification 14 and the DC voltage obtained by ACrectification sent via the resistor R3 and the Comp terminal.

According to the control IC configured as such, the FB terminal can beeliminated, and also, the diodes D3 and D4, the capacitor C3, and theresistor R4 shown in FIG. 7 can be eliminated. Although the filter 15 isprovided in FIG. 9, a source of the switching element Q1 may be directlyconnected to the inverting input terminal of the current-control erroramplification 14 without the filter 15.

Embodiment 4

FIG. 10 is a circuit diagram showing the inside of a control IC of aswitching power source device of Embodiment 4. The control IC shown inFIG. 10 does not have the resistors R1, R2, and R3 shown in FIG. 9, andis configured such that an average value of drain current of theswitching element Q1 is inputted to an inverting input terminal of acurrent-control error amplifier 14 via a filter 15 and that an output ofa Comp terminal is mixed with a forward voltage signal generated from anauxiliary winding W3 via a diode D5 and a capacitor C8 and convertedinto a current.

A voltage in accordance with a DC voltage obtained by AC rectificationis generated at the auxiliary winding W3. The DC voltage of a capacitorC8 generated at the auxiliary winding W3 obtained by the ACrectification and smoothing is outputted to a current mirror circuit 16via a det terminal. The current mirror circuit 16 converts the voltageof the capacitor C8 inputted via the det terminal into a current andoutputs the current to a non-inverting input terminal of a PWMcomparator 12.

The PWM comparator 12 is configured to generate a pulse signal bycomparing a triangular-wave signal from a triangular-wave circuit 13with an added output obtained by adding the current from the currentmirror circuit 16 which has been converted from the voltage inaccordance with the DC voltage obtained by AC rectification and anaverage value of drain current of the switching element Q1 from thecurrent-control error amplifier 14. Thereby, constant power control canbe performed.

According to the present invention, a voltage signal based on theaverage value of current flowing through the switching element and avoltage signal rectified and smoothed by the rectifying-smoothingcircuit are inputted to the control circuit. Hence, the feedback voltagevalue is constant irrespective of the input voltage value. Constantpower control can thus be performed. For this reason, constant powercontrol can be performed without needing a multiplier and a photocoupler, allowing provision of an inexpensive, small switching powersource device causing less change in the amount of luminous flux evenwhen the forward voltages VF vary.

What is claimed is:
 1. A switching power source device which is capableof performing constant power control, which converts AC power of an ACpower source into DC power and outputs the DC power, the devicecomprising: a first rectifying-smoothing circuit configured to output arectified-smoothed voltage signal obtained by rectifying and smoothingan AC voltage of the AC power source; a transformer having a primarywinding, a secondary winding, and an auxiliary winding; a switchingelement connected to the primary winding of the transformer; and acontrol circuit configured to turn the switching element on and offbased on a voltage signal which is based on an average value of currentflowing through the switching element and the rectified-smoothed voltagesignal from the first rectifying-smoothing circuit, wherein the controlcircuit has a feedback input terminal determining an on/off duty cycle,and the feedback input terminal receives the voltage signal which isbased on the average value of current flowing through the switchingelement and the rectified-smoothed voltage signal from the firstrectifying-smoothing circuit, further comprising; a current detectionresistor series-connected to one of main terminals of the switchingelement; a first resistor connected between an output terminal of thefirst rectifying-smoothing circuit and the feedback input terminal; afirst diode and a second diode which are connected in series withopposite orientations between the one of the main terminals of theswitching element and the feedback input terminal; a secondrectifying-smoothing circuit configured to output a rectified-smoothedvoltage signal obtained by rectifying and smoothing an AC voltagegenerated at auxiliary winding of the transformer; and a second resisterconfigured to provide a voltage proportional to the rectified-smoothedvoltage signal obtained by the second rectifying and smoothing circuitto a connection point between the first diode and the second diode.
 2. Aswitching power source device which is capable of performing constantpower control, which converts AC power of an AC power source into DCpower and outputs the DC power, the device comprising: arectifying-smoothing circuit configured to output a rectified-smoothedvoltage signal obtained by rectifying and smoothing an AC voltage of theAC power source; a transformer having a primary winding, a secondarywinding, and an auxiliary winding; a switching element connected to theprimary winding of the transformer; and a control circuit configured toturn the switching element on and off based on a voltage signal which isbased on an average value of current flowing through the switchingelement and the rectified-smoothed voltage signal from therectifying-smoothing circuit, wherein the control circuit includes anerror amplifier configured to amplify an error voltage between areference voltage and the voltage signal which is based on the averagevalue of current flowing through the switching element, and a PWMcomparator configured to generate a pulse signal for turning theswitching element on and off, by comparing a triangular-wave signal withan added output obtained by adding an output of the error amplifier to avoltage which is generated at the auxiliary winding of the transformerand proportional to the rectified-smoothed voltage signal.
 3. A controlIC which is capable of performing constant power control comprising: aswitching element connected to a primary winding of a transformer; anerror amplifier configured to amplify an error voltage between areference voltage and a voltage signal which is based on an averagevalue of current flowing through the switching element; and a PWMcomparator configured to generate a pulse signal for turning theswitching element on and off, by comparing a triangular-wave signal withan added output obtained by adding an output of the error amplifier to avoltage which is generated at an auxiliary winding of the transformerand proportional to a rectified-smoothed voltage signal obtained byrectification and smoothing of an AC voltage.